This invention relates in general to thickness measurement of structures and, in particular, to a system for measuring film thickness through laser induced acoustic pulse-echo using non-contact interferometry.
Ellipsometry is a powerful technique for film thickness measurement in semiconductor processing. In cases where the film under examination is transparent to the illuminating radiation, ellipsometry can measure films down to one monolayer thick (3-10 Angstroms). However, ellipsometry fails in cases where the film under examination is opaque. Metallic films, which play a major role in integrated circuit fabrication, fall into this category. Optical radiation is absorbed within the first few tens to hundreds of Angstroms of the film, depending on the wavelength and material under examination. For example, using green radiation at a wavelength of 0.5 micron in aluminum, the absorption length is less than 70 Angstroms. At longer wavelengths, and in particular at infra-red wavelengths, this situation gets better, but still ellipsometry cannot provide the full solution with reference to metallic films or other optically opaque films.
Time resolved pulse-echo ultrasound is a well known technique for thickness measurement in situations where the thickness of interest is a few millimeters or at least tens of microns. For films used in semiconductor processing, one needs extremely short pulses so that the surface echo can be time resolved. Such pulses can be generated by short laser pulses and the general area of this art is known as photoacoustics. The physical processes involved is as follows: a short laser pulse is absorbed within one absorption length from the surface, causing a rise in local temperature of the surface. Through the temperature coefficient of expansion (expansivity) the film undergoes thermal stresses leading to an elastic pulse which propagates across the film at the speed of sound. Given the velocity of sound in the film, if one measures the time of flight across the film, one can compute the film thickness. The key-remaining issue, is therefore, the detection of the acoustic disturbance once it bounces back from the rear side of the film and reaches the front surface.
Reference is made to the work of investigators at the Brown University in U.S. Pat. No. 4,710,030. The patent states that once a stress pulse is reflected from the rear side of the film and reaches the surface, it changes the optical constants of the surface and near surface. It can be shown that these changes can be as low as a few parts in 10.sup.6, depending on both the elastic and electronic properties of the film. The change in the optical constants of the surface leads to a change in reflectivity which is detected by monitoring the intensity in a "probe" beam which also illuminates the surface. Given that the change in optical constants is small, the method patented by workers at Brown University, at best, lacks sensitivity.
It is, therefore, desirable to provide improved techniques for film thickness measurements.